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Enhanced Performance with the Incorporation of Organo-metal Trihalide Perovskite in Nanostructured ZnO Solar Cell
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 141 (2016) 1 – 6
MRS Singapore – ICMAT Symposia Proceedings 8th International Conference on Materials for Advanced Technologies
Enhanced Performance with the incorporation of Organo-metal trihalide perovskite in nanostructured ZnO Solar Cell S. Kumar, S. Pradhan, and A. Dhar* Department of Physics, IIT Kharagpur, Kharagpur-721302, India
Abstract The tremendous developments in the field of organic photovoltaics (efficiency>12%) in recent years has opened up a new possibility for the commercialization of low cost, flexible, and third generation photovoltaics. However, the rapid degradation of the organic materials and their short exciton diffusion lengths are major hindrance for the complete development of organic solar cells for commercial use. The concept of organic/inorganic hybrid solar cell which combines the advantages of both organic and inorganic materials are readily applicable for low cost, low temperature, high yield, and solution processable techniques but suffers from low fundamental efficiency. Recently, a new class of orgnanic/inorganic hybrid material known as organometal trihalide perovskites have emerged which exhibit the properties of good absorbtion, free charge carrier generation and efficient transport of the generated carriers while maintaining the primary requirements of organic electronics viz. low temperature and solution processability. In our present work, we have demonstrated the enhancement in device efficiency by incorporating organometal trihalide perovskite (CH3NH3PbI3) into ZnO nanostructured/MDMO-PPV inverted solar cell. The devices with perovskite absorbers showed a photocoversion efficiency of 1.97 % in contrast to mere 0.06 % obtained for ZnO/MDMO-PPV devices. ©2016 2015The TheAuthors. Authors. Published Elsevier © Published by by Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT. Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT Keywords: Methyl ammonium lead iodide, Zinc oxide, Hybrid solar cell
* Corresponding author. Tel.: +91 322228380; fax: +91 3222255303. E-mail address:[email protected]
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT
doi:10.1016/j.proeng.2015.08.1126
2
S. Kumar et al. / Procedia Engineering 141 (2016) 1 – 6
1. Introduction Since last two decades, organic polymer based solar cells have drawn significant amount of deliberation among the researchers for their usefulness in fabricating eco-friendly, cheap, and large area solar energy panels. Recently, photoconversion efficiencies (PCE) exceeding 12% have been reported by some groups for organic bulk heterojunction solar cells [1] which show organic based solar cells have strong potential for replacing the conventional Si-based inorganic solar cells. However, these types of third generation solar cells possess various problems before being taken up by the companies for commercialization. Some of the major problems being phase segregation, low carrier mobility of organic semiconductors, short exciton diffusion lengths, and selective absorption in the visible region [2]. Many groups have tried various means of overcoming the present demerits of organic solar cells out of which using nanostructured metal oxide/conjugated polymer hybrid solar cell is very well established [34]. Nevertheless, these kinds of solar cell also suffer from lower device efficiency because of their inherent demerits like poor infiltration of polymer inside nanostructures and recombination losses at the heterojunction interface, and inability to absorb in the broad region of solar spectrum [5-7]. From past few years a new class of materials, organo-metal trihalide have emerged which are consistently showing high performance when used in solar cell with various device architectures [8]. They are reported to have excellent absorption in broad visible region, long range electron-hole diffusion lengths, and ambipolar charge transport due to which many device applications have been proposed including dye-sensitized cells, field effect transistors, and photodetectors. Herein, we have studied the enhancement of efficiency in nanostructured ZnO/MDMO-PPV based hybrid solar cell by sensitizing the ZnO nanostructures with CH3NH3PbI3 perovskite material. The effect of CH3NH3PbI3 in increasing the device absorption efficiency and the heterojunction properties has been studied. 2. Experimental procedure The schematic of the device architecture is shown in Fig. 1. Indium tin oxide (ITO) coated glass substrate were cleaned sequentially in acetone, proponal, and DI water and purged with nitrogen. ZnO base layer was formed over the cleaned ITO substrates using Zince acetate dehydrate (>98 %) in methanol and was annealed at 150 ˚C for 30 minutes. The nanostructures of ZnO were grown by conventional chemical bath deposition technique (CBD) [9]. The ZnO base layer coated ITO substrates were immersed for 45 minutes in aqueous solution containing Zinc nitrate hexahydrate (98%) and Hexamethylene tetramine (99 %) kept at 90 ˚C.
Fig. 1: Schematic of the solar cell device architecture.
Methylammonium iodide (CH3NH3I) was synthesized according to reported procedures [10]. 0.3 M Methylamine (CH3NH2) solution (33 wt% in absolute ethanol) was reacted with 0.3 M hydroiodic acid (HI) (57 wt % in water) under stirring at 0 ˚C for 2 hours. The solution was crystallized using a rotary evaporator at 60 ˚C for 3 hours to obtain CH3NH3I. Methylammonium lead iodide (CH3NH3PbI3) was synthesized by mixing CH3NH3I powder and lead (II) iodide (PbI2) in anhydrous dimethylformamide (DMF) in 1:3 weight ratio. The precursor
S. Kumar et al. / Procedia Engineering 141 (2016) 1 – 6
solution (9 wt %) was stirred overnight before spincoating it onto the ZnO nanostructures and the substrates were heated at 100 ˚C for 2 mins. Hole transporting MDMO-PPV (12mg/ml) dissolved in 1,2-dichlorobenzene was spincoated at 1000 rpm for 45 seconds. Finally, ~5 nm Molybdenum trioxide (MoO3) buffer layer was deposited by thermal evaporation prior to the top electrode (aluminum) deposition. The electrical measurements were done using Keithley-4200 SCS unit and AM 1.5 solar irradiation under ambient atmosphere. The absorption efficiencies of the fabricated devices with and without perovskite layer were measured using UV-Vis- NIR spectrometer (Avaspec-3648 fiber optic spectrometer. Field emission scanning electron microscopy (FESEM) was done using Zeiss SUPRA 40 field-emission microscope to view the bare ZnO nanostructures and the perovskite coated ZnO nanostructures. 3. Results and Discussions The representative energy band diagram of nanostructured ZnO/CH3NH3PbI3/MDMO-PPV solar device is shown in Fig. 2. The highest occupied molecular orbital (HOMO) and lowest occupied molecular orbital (LUMO) of the perovskite material fits well between the energy levels of ZnO and MDMO-PPV as can been seen in the diagram below. The suitable energy levels help efficient separation of the generated exciton at the interface.
Fig. 2: Energy level diagram of the device.
Figure 3 (a) shows the FESEM image of the ZnO base layer. The base layer is uniform which is very crucial in reducing the device leakage current and obtain high rectification ratio of the diode. The top-view of the ZnO nanostructures is shown in Fig. 3 (b). The average diameter and separation between the ZnO nanostructures is ~50 nm and ~100 nm. Fig. 3 (c) shows the top view of perovskite coated ZnO nanostructures. AFM image of ZnO base layer (Fig. 4 (a)) shows the uniformity of the base layer with rms roughness of 2.66 nm. The average grain size was measured to be ~30 nm. Fig. 4 (b) depicts the AFM image of CH3NH3PbI3 film coated on glass and annealed at 100 ˚C. The grain size in perovskite is ~400 nm with rms roughness of 44.4 nm which is considerably larger than ZnO base layer. The absorption spectra of different layers used in the solar device is shown in Fig. 5 (a). The MDMO-PPV polymer has narrow peak between 400nm and 600 nm with maximum absorbance at 500 nm. The perovskite layer possesses broad absorbance in the visible region from 400 nm to 800 nm owing to which the absorption efficiency of the combined CH3NH3PbI3/MDMO-PPV device increased in the whole visible region as depicted in the plot.
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S. Kumar et al. / Procedia Engineering 141 (2016) 1 – 6
(a)
(c)
(b)
Fig. 3: FESEM image of (a) ZnO base layer (b) ZnO nanorods (c) CH3NH3PbI3/ZnO nanorods
(b)
(a)
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CH3NH3PbI3/MDMO-PPV CH3NH3PbI3
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MDMO-PPV
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0.0 400
500
600
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700
800
(b) Current Density (mA/cm2)
(a)
Normalized Absorbance (Arb. Units)
Fig. 4: AFM image of (a) ZnO base layer (b) CH3NH3PbI3 5
with CH3NH3PbI3 without CH3NH3PbI3 0
-5
-10
-0.4
-0.2
0.0
0.2
0.4
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Fig. 5: (a) Absorbance spectra of MDMO-PPV and CH3NH3PbI3 (b) J-V characteristics under AM 1.5 solar irradiation
The measured current density vs. voltage plots for ZnO/MDMO-PPV and ZnO/CH3NH3PbI3/MDMO-PPV devices measured under AM 1.5 solar irradiation are shown in Fig. 5 (b). The devices without perovskite interlayer showed very nominal performance having short circuit current density (Jsc) of 0.45 mA/cm2 and open circuit voltage (Voc) of 0.4 V. The calculated fill factor (FF) for the device was 0.32. The addition of perovskite layer between ZnO nanostructures and MDMO-PPV hole transporting layer increased the value of Jsc to 8.64 mA/cm2 and Voc to 0.6 V. the fill factor of the device also increased to 0.34. All of the above effects resulted in an overall improvement in device efficiency from 0.06 % to 1.97 %.
S. Kumar et al. / Procedia Engineering 141 (2016) 1 – 6
In literature, we find many reports on efficiency enhancement of nanostructured ZnO/conjugated polymer devices by using various techniques such as chemical surface modification of ZnO nanostructures, use of molecular dipoles, incorporation of third component between ZnO and the conjugate polymer, etc [11-14]. Olson et al. reported a significant enhancement in device performance from 0.53% to 2.03% of ZnO nanorod/P3HT hybrid devices by introduction of phenyl C61-butyric acid methyl ester (PCBM) into the device architecture [15]. A method of similar kind of efficiency enhancement was reported on F-doped (FTO)-coated ZnO nanorod arrays/P3HT devices using CdS nanocrystals by Chen et al [16]. They reported a increment in efficiency from 0.37% to 1.8% with CdS incorporation. They also studied the effect of ZnO nanorod array on the device performance. An enhancement of 15 times was observed in ZnO/MDMO-PPV devices (0.02% to 0.29%) by treating the ZnO nanostructures with ammonia solution [9]. The reason for the performance enhancement was attributed to the better infiltration of MDMO-PPV solution into ZnO nanostructures due to ammonia surface modification of ZnO nanostructures and better charge separation due to the interfacial dipoles caused by ammonia molecules. Ultra long PTCDI-C8 nanoribbons were used as ZnO nanorods surface modifiers leading to increase in photovoltaic efficiency from 0.29% to 0.46% in ZnO/MDMO-PPV devices [17]. Increase in the device absorption efficiency due to incorporation of PTCDI-C8 nanoribbons and the better charge separation and transport in the device were reported to be the major reasons behind the performance enhancement. There have reports on the effect of perovskite kind of materials on the photovoltaic performance of nanostructured ZnO photovolotaic devices. Recently, Dunn et al reported the use of bismuth ferrite perovskite (BFO) in enhancing the photovoltaic efficiency from 0.1% to 0.38% in ZnO nanostructures/copper thiocyanate (CuSCN) based photocells [18]. Suppression in charge recombination and enhanced interfacial properties were reported to be the major reasons behind the improvement in the performance. An efficiency of 3.83% was reported ZnO/CH3NH3PbI3/pristine spiro-MeOTAD was reported by Ma and group [19]. They studied the effect of hole transporting layer (HTMs) on the device efficiency and reported the validity of ionic liquid BuPyIm-TFSI as a dual functional additive in enhancing the HTM conductivity and suppressing the charge recombination. Thus, based on the works mentioned above, our report on the efficiency enhancement by incorporating CH3NH3PbI3 interlayer in nanostructured ZnO/MDMO-PPV device is comparable in terms of the increment of the device performance. The increased absorption of the device which resulted in increased number of generated exciton, suitable positioning of the perovskite energy levels and the inherent property of long range exciton diffusion collectively resulted in the enhanced performance of hybrid nanostructured ZnO/CH3NH3PbI3/MDMO-PPV solar device. Further optimization of the various layers and use of better hole transporting layers may result further improvement in device efficiency. 4. Conclusions We have shown the enhancement in the device performance of nanostructured ZnO/conjugated polymer solar cell by incorporating a CH3NH3PbI3 perovskite material which absorbs in the whole visible region. The photoconversion efficiency (݅) of the device increased from 0.06 % to 1.97 %.The increment in the device efficiency was attributed to enhanced absorption efficiency of the device and probably due to better infiltration of the perovskite into the nanostructures in contrast to the MDMO-PPV. Further studies on the carrier recombination losses and efficiency of charge transfer at the interface can be made through photoluminescence experiments which can give a better insight on the role of perovskite in enhancing the device performance. References [1] See http://www.heliatek.com/ [2] H. Hung, J. Huang, Organic and Hybrid Solar Cells, Springer International Publishing, Switzerland 2014. [3] P. Ravirajan, A. M. Peiro, M. K. Nazeeruddin, M. Graetzel, D. D. C. Bradley, J. R. Durrant, and J. Nelson, Hybrid Polymer/Zinc Oxide Photovoltaic Devices with Vertically Oriented ZnO Nanorods and an Amphiphilic Molecular Interface Layer, J. Phys. Chem. B 110 (2006) 7635. [4] T. H. Lee, H. J. Sue, and X. Cheng, ZnO and conjugated polymer bulk heterojunction solar cells containing ZnO nanorod photoanode, Nanotechnology 22 (2011) 285401. [5] P. Ravirajan, S. A. Haque, J. R. Durrant, D. Poplavskyy, D. D. C. Bradley, and J. Nelson, Hybrid nanocrystalline TiO2 solar cells with a fluorene–thiophene copolymer as a sensitizer and hole conductor, J. Appl. Phys. 95 (2004) 1473. [6] S. R. Cowan, N. Banerji, W. L. Leong, and A. J. Heeger, Charge formation, recombination, and sweepǦout dynamics in organic solar cells, Adv. Funct. Mater. 22 (2012) 1116. [7] X. Fan, M. Zhang, X. Wang, F. Yang , and X. Meng, Recent progress in organic–inorganic hybrid solar cells, J. Mater. Chem. A 1 (2013) 8694. [8] A. Kojima, K. Teshima, Y. Shirai, and T. Miyasaka, Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells, J. Am. Chem. Soc. 131 (2009) 6050.
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[9] S. Pradhan, S. Karak, and A. Dhar, Enhancing the performance of nanostructured zinc oxide/polymer-based hybrid solar cells using ammonia as a structural and interfacial modifier, J. Phys. D: Appl. Phys. 45 (2012) 235104. [10] J. Qiu, Y. Qiu, K. Yan, M. Zhong, C. Mu, H. Yan, S.Yang, All-solid-state hybrid solar cells based on a new organometal halide perovskite sensitizer and one-dimensional TiO2 nanowire arrays, Nanoscale 5 (2013) 3245. [11] C. Goh, S. R. Scully, M. D. McGehee, Effect of molecular interface modificationin hybrid organic-inorganic photovoltaic cells, Journal of Applied Physics 101 (2007) 114503 (1–12). [12] C. T. Chen, F .C. Hsu, S. W. Kuan, Y. F. Chen, Effect of C60 on the ZnO nanorod surface in organic-inorganic hybrid photovoltaics, Solar Energy Materials andSolar Cells 95 (2011) 740. [13] J. S. Huang, C.Y. Chou, C. F. Lin, Enhancing performance of organic–inorganichybrid solar cells using a fullerene interlayer from allsolution processing, SolarEnergy Materials and Solar Cells 94 (2010) 182. [14] P. Atienzar, T. Ishwara, B. N. Illy, M. P. Ryan, B. C. O’Regan, J. R. Durrant, J. Nelson, Control of photocurrent generation in polymer/ZnO nanorod solar cells by usinga solution-processed TiO2 overlayer, The Journal of Physical Chemistry Letters 1 (2010) 708. [15] D. C. Olson, J. Piris, R. T. Collins, S. E. Shaheen and D. S. Ginley, Hybrid photovoltaic devices of polymer and ZnO nanofiber composites, Thin Solid Films 26 (2006) 496. [16] H. C. Liao, C. C. Lin, Y.W. Chen, T. C. Liu, and S.Y. Chen, Improvement in photovoltaic performance for hybrid P3HT/elongated CdS nanocrystals solar cells with F-doped SnO2 arrays, J. Mater. Chem. 20 (2010) 5429. [17] S. Pradhan, A. Dhar, Improvement of the nanostructured zinc oxide/polymer based solarcell efficiency through the incorporation ofN,N dioctyl-3,4,9,10-perylenedicarboximide nanoribbons ascharge mediator, Synthetic Metals 176 (2013) 116. [18] L. Loh, J. Briscoe, and S. Dunn, Enhanced performance with bismuth ferrite perovskite in ZnO nanorod solid state solar cells, Nanoscale 6 (2014) 7072. [19] H. Zhang, Y. Shi, F. Yan, L. Wang, K. Wang, Y. Xing, Q. Donga, and T. Ma, A dual functional additive for the HTM layer in perovskite solar cells, Chem. Commun. 50 (2014) 5020.
ScienceDirect Procedia Engineering 141 (2016) 1 – 6
MRS Singapore – ICMAT Symposia Proceedings 8th International Conference on Materials for Advanced Technologies
Enhanced Performance with the incorporation of Organo-metal trihalide perovskite in nanostructured ZnO Solar Cell S. Kumar, S. Pradhan, and A. Dhar* Department of Physics, IIT Kharagpur, Kharagpur-721302, India
Abstract The tremendous developments in the field of organic photovoltaics (efficiency>12%) in recent years has opened up a new possibility for the commercialization of low cost, flexible, and third generation photovoltaics. However, the rapid degradation of the organic materials and their short exciton diffusion lengths are major hindrance for the complete development of organic solar cells for commercial use. The concept of organic/inorganic hybrid solar cell which combines the advantages of both organic and inorganic materials are readily applicable for low cost, low temperature, high yield, and solution processable techniques but suffers from low fundamental efficiency. Recently, a new class of orgnanic/inorganic hybrid material known as organometal trihalide perovskites have emerged which exhibit the properties of good absorbtion, free charge carrier generation and efficient transport of the generated carriers while maintaining the primary requirements of organic electronics viz. low temperature and solution processability. In our present work, we have demonstrated the enhancement in device efficiency by incorporating organometal trihalide perovskite (CH3NH3PbI3) into ZnO nanostructured/MDMO-PPV inverted solar cell. The devices with perovskite absorbers showed a photocoversion efficiency of 1.97 % in contrast to mere 0.06 % obtained for ZnO/MDMO-PPV devices. ©2016 2015The TheAuthors. Authors. Published Elsevier © Published by by Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT. Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT Keywords: Methyl ammonium lead iodide, Zinc oxide, Hybrid solar cell
* Corresponding author. Tel.: +91 322228380; fax: +91 3222255303. E-mail address:[email protected]
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT
doi:10.1016/j.proeng.2015.08.1126
2
S. Kumar et al. / Procedia Engineering 141 (2016) 1 – 6
1. Introduction Since last two decades, organic polymer based solar cells have drawn significant amount of deliberation among the researchers for their usefulness in fabricating eco-friendly, cheap, and large area solar energy panels. Recently, photoconversion efficiencies (PCE) exceeding 12% have been reported by some groups for organic bulk heterojunction solar cells [1] which show organic based solar cells have strong potential for replacing the conventional Si-based inorganic solar cells. However, these types of third generation solar cells possess various problems before being taken up by the companies for commercialization. Some of the major problems being phase segregation, low carrier mobility of organic semiconductors, short exciton diffusion lengths, and selective absorption in the visible region [2]. Many groups have tried various means of overcoming the present demerits of organic solar cells out of which using nanostructured metal oxide/conjugated polymer hybrid solar cell is very well established [34]. Nevertheless, these kinds of solar cell also suffer from lower device efficiency because of their inherent demerits like poor infiltration of polymer inside nanostructures and recombination losses at the heterojunction interface, and inability to absorb in the broad region of solar spectrum [5-7]. From past few years a new class of materials, organo-metal trihalide have emerged which are consistently showing high performance when used in solar cell with various device architectures [8]. They are reported to have excellent absorption in broad visible region, long range electron-hole diffusion lengths, and ambipolar charge transport due to which many device applications have been proposed including dye-sensitized cells, field effect transistors, and photodetectors. Herein, we have studied the enhancement of efficiency in nanostructured ZnO/MDMO-PPV based hybrid solar cell by sensitizing the ZnO nanostructures with CH3NH3PbI3 perovskite material. The effect of CH3NH3PbI3 in increasing the device absorption efficiency and the heterojunction properties has been studied. 2. Experimental procedure The schematic of the device architecture is shown in Fig. 1. Indium tin oxide (ITO) coated glass substrate were cleaned sequentially in acetone, proponal, and DI water and purged with nitrogen. ZnO base layer was formed over the cleaned ITO substrates using Zince acetate dehydrate (>98 %) in methanol and was annealed at 150 ˚C for 30 minutes. The nanostructures of ZnO were grown by conventional chemical bath deposition technique (CBD) [9]. The ZnO base layer coated ITO substrates were immersed for 45 minutes in aqueous solution containing Zinc nitrate hexahydrate (98%) and Hexamethylene tetramine (99 %) kept at 90 ˚C.
Fig. 1: Schematic of the solar cell device architecture.
Methylammonium iodide (CH3NH3I) was synthesized according to reported procedures [10]. 0.3 M Methylamine (CH3NH2) solution (33 wt% in absolute ethanol) was reacted with 0.3 M hydroiodic acid (HI) (57 wt % in water) under stirring at 0 ˚C for 2 hours. The solution was crystallized using a rotary evaporator at 60 ˚C for 3 hours to obtain CH3NH3I. Methylammonium lead iodide (CH3NH3PbI3) was synthesized by mixing CH3NH3I powder and lead (II) iodide (PbI2) in anhydrous dimethylformamide (DMF) in 1:3 weight ratio. The precursor
S. Kumar et al. / Procedia Engineering 141 (2016) 1 – 6
solution (9 wt %) was stirred overnight before spincoating it onto the ZnO nanostructures and the substrates were heated at 100 ˚C for 2 mins. Hole transporting MDMO-PPV (12mg/ml) dissolved in 1,2-dichlorobenzene was spincoated at 1000 rpm for 45 seconds. Finally, ~5 nm Molybdenum trioxide (MoO3) buffer layer was deposited by thermal evaporation prior to the top electrode (aluminum) deposition. The electrical measurements were done using Keithley-4200 SCS unit and AM 1.5 solar irradiation under ambient atmosphere. The absorption efficiencies of the fabricated devices with and without perovskite layer were measured using UV-Vis- NIR spectrometer (Avaspec-3648 fiber optic spectrometer. Field emission scanning electron microscopy (FESEM) was done using Zeiss SUPRA 40 field-emission microscope to view the bare ZnO nanostructures and the perovskite coated ZnO nanostructures. 3. Results and Discussions The representative energy band diagram of nanostructured ZnO/CH3NH3PbI3/MDMO-PPV solar device is shown in Fig. 2. The highest occupied molecular orbital (HOMO) and lowest occupied molecular orbital (LUMO) of the perovskite material fits well between the energy levels of ZnO and MDMO-PPV as can been seen in the diagram below. The suitable energy levels help efficient separation of the generated exciton at the interface.
Fig. 2: Energy level diagram of the device.
Figure 3 (a) shows the FESEM image of the ZnO base layer. The base layer is uniform which is very crucial in reducing the device leakage current and obtain high rectification ratio of the diode. The top-view of the ZnO nanostructures is shown in Fig. 3 (b). The average diameter and separation between the ZnO nanostructures is ~50 nm and ~100 nm. Fig. 3 (c) shows the top view of perovskite coated ZnO nanostructures. AFM image of ZnO base layer (Fig. 4 (a)) shows the uniformity of the base layer with rms roughness of 2.66 nm. The average grain size was measured to be ~30 nm. Fig. 4 (b) depicts the AFM image of CH3NH3PbI3 film coated on glass and annealed at 100 ˚C. The grain size in perovskite is ~400 nm with rms roughness of 44.4 nm which is considerably larger than ZnO base layer. The absorption spectra of different layers used in the solar device is shown in Fig. 5 (a). The MDMO-PPV polymer has narrow peak between 400nm and 600 nm with maximum absorbance at 500 nm. The perovskite layer possesses broad absorbance in the visible region from 400 nm to 800 nm owing to which the absorption efficiency of the combined CH3NH3PbI3/MDMO-PPV device increased in the whole visible region as depicted in the plot.
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S. Kumar et al. / Procedia Engineering 141 (2016) 1 – 6
(a)
(c)
(b)
Fig. 3: FESEM image of (a) ZnO base layer (b) ZnO nanorods (c) CH3NH3PbI3/ZnO nanorods
(b)
(a)
1.0
CH3NH3PbI3/MDMO-PPV CH3NH3PbI3
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500
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700
800
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(a)
Normalized Absorbance (Arb. Units)
Fig. 4: AFM image of (a) ZnO base layer (b) CH3NH3PbI3 5
with CH3NH3PbI3 without CH3NH3PbI3 0
-5
-10
-0.4
-0.2
0.0
0.2
0.4
0.6
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Fig. 5: (a) Absorbance spectra of MDMO-PPV and CH3NH3PbI3 (b) J-V characteristics under AM 1.5 solar irradiation
The measured current density vs. voltage plots for ZnO/MDMO-PPV and ZnO/CH3NH3PbI3/MDMO-PPV devices measured under AM 1.5 solar irradiation are shown in Fig. 5 (b). The devices without perovskite interlayer showed very nominal performance having short circuit current density (Jsc) of 0.45 mA/cm2 and open circuit voltage (Voc) of 0.4 V. The calculated fill factor (FF) for the device was 0.32. The addition of perovskite layer between ZnO nanostructures and MDMO-PPV hole transporting layer increased the value of Jsc to 8.64 mA/cm2 and Voc to 0.6 V. the fill factor of the device also increased to 0.34. All of the above effects resulted in an overall improvement in device efficiency from 0.06 % to 1.97 %.
S. Kumar et al. / Procedia Engineering 141 (2016) 1 – 6
In literature, we find many reports on efficiency enhancement of nanostructured ZnO/conjugated polymer devices by using various techniques such as chemical surface modification of ZnO nanostructures, use of molecular dipoles, incorporation of third component between ZnO and the conjugate polymer, etc [11-14]. Olson et al. reported a significant enhancement in device performance from 0.53% to 2.03% of ZnO nanorod/P3HT hybrid devices by introduction of phenyl C61-butyric acid methyl ester (PCBM) into the device architecture [15]. A method of similar kind of efficiency enhancement was reported on F-doped (FTO)-coated ZnO nanorod arrays/P3HT devices using CdS nanocrystals by Chen et al [16]. They reported a increment in efficiency from 0.37% to 1.8% with CdS incorporation. They also studied the effect of ZnO nanorod array on the device performance. An enhancement of 15 times was observed in ZnO/MDMO-PPV devices (0.02% to 0.29%) by treating the ZnO nanostructures with ammonia solution [9]. The reason for the performance enhancement was attributed to the better infiltration of MDMO-PPV solution into ZnO nanostructures due to ammonia surface modification of ZnO nanostructures and better charge separation due to the interfacial dipoles caused by ammonia molecules. Ultra long PTCDI-C8 nanoribbons were used as ZnO nanorods surface modifiers leading to increase in photovoltaic efficiency from 0.29% to 0.46% in ZnO/MDMO-PPV devices [17]. Increase in the device absorption efficiency due to incorporation of PTCDI-C8 nanoribbons and the better charge separation and transport in the device were reported to be the major reasons behind the performance enhancement. There have reports on the effect of perovskite kind of materials on the photovoltaic performance of nanostructured ZnO photovolotaic devices. Recently, Dunn et al reported the use of bismuth ferrite perovskite (BFO) in enhancing the photovoltaic efficiency from 0.1% to 0.38% in ZnO nanostructures/copper thiocyanate (CuSCN) based photocells [18]. Suppression in charge recombination and enhanced interfacial properties were reported to be the major reasons behind the improvement in the performance. An efficiency of 3.83% was reported ZnO/CH3NH3PbI3/pristine spiro-MeOTAD was reported by Ma and group [19]. They studied the effect of hole transporting layer (HTMs) on the device efficiency and reported the validity of ionic liquid BuPyIm-TFSI as a dual functional additive in enhancing the HTM conductivity and suppressing the charge recombination. Thus, based on the works mentioned above, our report on the efficiency enhancement by incorporating CH3NH3PbI3 interlayer in nanostructured ZnO/MDMO-PPV device is comparable in terms of the increment of the device performance. The increased absorption of the device which resulted in increased number of generated exciton, suitable positioning of the perovskite energy levels and the inherent property of long range exciton diffusion collectively resulted in the enhanced performance of hybrid nanostructured ZnO/CH3NH3PbI3/MDMO-PPV solar device. Further optimization of the various layers and use of better hole transporting layers may result further improvement in device efficiency. 4. Conclusions We have shown the enhancement in the device performance of nanostructured ZnO/conjugated polymer solar cell by incorporating a CH3NH3PbI3 perovskite material which absorbs in the whole visible region. The photoconversion efficiency (݅) of the device increased from 0.06 % to 1.97 %.The increment in the device efficiency was attributed to enhanced absorption efficiency of the device and probably due to better infiltration of the perovskite into the nanostructures in contrast to the MDMO-PPV. 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